1. Field of the Invention
The present invention generally relates to an exhaust pipe opening and closing apparatus. More particularly, the present invention relates to the exhaust pipe opening and closing apparatus which selectively opens and closes a main flow passage of the exhaust pipe and a bypass flow passage branching from the main flow passage.
2. Description of the Related Art
In a system for purifying exhaust gas emitted from an automotive engine, a catalytic conversion system in which a carrier carrying a noble metal such as platinum, rhodium or the like as a catalyst is provided in an exhaust pipe and purifies HC, CO and NOx or the like in the exhaust gas by oxidation or oxidation and reduction reaction.
In the catalytic conversion system, the catalyst needs to be heated up to a catalytic activation temperature of, for example, 300.degree. to 400.degree. C. or more. However, in general, since a catalytic heating method with exhaust gas is used, the catalyst does not reach the catalytic activation temperature, and therefore little exhaust gas can be purified.
In order to heat up the catalyst to the catalytic activation temperature as early as possible, the catalyst is disposed next to an automotive engine so as to be exposed to the exhaust gas therefrom, a material used for the carrier carrying the catalyst is changed from ceramics to a metal having a high heat conductivity, or a heater is installed at the carrier so as to be heated forcibly.
In order to restrict the exhaust gas from automobiles, purification ratio of toxic gas components such as HC, CO and NOx needs to be improved more. Therefore, the catalyst is disposed at a position close to the engine so that the exhaust gas can be purified just after the engine is started. However, when the catalyst is provided close to the engine, the catalyst is exposed to the exhaust gas with high temperature. Since the catalyst is generally decayed early at high temperature, the purification ratio is lowered.
As disclosed in the Japanese Patent Application Laid-Open No. 4-60108, the exhaust pipe is diverged into two parts near the engine to form a main flow passage and a bypass flow passage. A main catalyst is disposed on the downstream side of a portion in which the bypass flow passage joins the main flow passages again, and a heater-provided catalyst is disposed at the bypass flow passage. A switching valve (opening and closing device) of a rotatably movable type is provided at a branch portion of the exhaust pipe near the engine. When the exhaust gas is at a low temperature immediately after the start of the engine, the main flow passage is closed and the bypass flow passage is opened, and thereby the exhaust gas is flowed into the bypass flow passage and purified by the heater-provided catalyst quickly heated up to the catalytic activation temperature. After the exhaust gas is heated up and the main catalyst is sufficiently heated by the exhaust gas, the bypass flow passage is closed and the main flow passage is opened by the switching valve, and therefore, the exhaust gas is flowed into the main flow passage and purified by the main catalyst.
The switching valve provided in the exhaust pipe and exposed to the exhaust gas with high temperature needs to have a sufficient heat resistance. Particularly, a main shaft of the switching valve is deformed into an arch shape in an axial direction by being exposed to high temperature and a cross section of the switching valve is strained into an elliptic shape. Thus, the main shaft inserted into a bearing hole is deformed into the arch shape. Hence, a middle portion of the main shaft contacts an inner periphery of the bearing hole in addition to having two ends of the main shaft supported and therefore, what is called "prize" is produced. Or the cross section of the main shaft is strained into the elliptic shape, the main shaft forcibly contacts the inner periphery of the bearing hole. Therefore, what is called "prize" is produced. As a result, the smooth operation of the main shaft is disturbed. Then, the opening and closing operation of the switching valve is no longer performed completely. When the switching valve closes the main flow passage and closes the bypass flow passage, the opening and closing operation is incomplete. Therefore, the exhaust gas leaks into the main flow passage and is discharged into an atmosphere without being purified by the heater-provided catalyst.
Furthermore, in order to connect the main shaft to an external driving device, the main shaft is constructed so as to protrude outward from the exhaust pipe through the bearing hole. At that time, since the main shaft may be deformed and strained at high temperature, a wide clearance needs to be made between the bearing hole and the main shaft. Thus, the exhaust gas leaks into the atmosphere through the clearance. When the clearance is made to narrower, the main shaft has malfunction or the like due to a small deformation or a small strain.